A ceramic heater-type glow plug which is used for assisting the start-up of a diesel engine is, in general, configured as follows. A rear portion side of a ceramic heater is held inside a metallic outer sleeve in a state that a heat generating portion formed on a distal end of the ceramic heater projects to the outside of the metallic outer sleeve. A rear end of the metallic outer sleeve is inserted and fixed in the inside of a front end portion of a cylindrical housing which is a fitting for mounting the plug on a cylinder head of an engine. One electrode (negative electrode) of the ceramic heater is taken out to an outer surface of a heater body and is electrically connected to an inner surface of the metallic outer sleeve and, at the same time, the other electrode (positive electrode) is taken out to the outside from a rear end side of the heater body by way of an electrode lead-out fitting and is electrically connected to an external connection terminal which is fixed to the rear end portion of the housing by way of an insulating member.
In the above-mentioned ceramic heater-type glow plug, there has been proposed a technique which grinds a rear end portion of a ceramic heater thus forming a small-diameter portion for connecting a positive electrode side of the ceramic heater and an electrode lead-out fitting (see patent document 1, for example).
Recently, to satisfy the restriction of an exhaust gas emission, a combustion system of a diesel engine has been shifted from a type which includes a sub combustion chamber to a directly injecting type, that is, a direct injection type and, further, the development of a multi-valve formation has been in progress. A glow plug used in such a direct-injection-type diesel engine faces a main combustion chamber through a wall surface of a cylinder head and hence, it is necessary to elongate a total length of the plug and also to reduce a diameter thereof compared to a glow plug which preheats the sub combustion chamber.
Further, it is necessary to increase a thickness of the cylinder head to ensure strength of the cylinder head and hence, an insertion hole for mounting the glow plug is required to be extremely fine and elongated whereby it is necessary to make the glow plug extremely elongated correspondingly.
To satisfy the above-mentioned requirement on the elongation of the glow plug and also to realize the reduction of cost by shortening a total length of the ceramic heater, there has been already known a glow plug having the structure in which a ceramic heater is fixed to one end side of a metallic outer sleeve in a state that a distal-end heat generating portion of the ceramic heater in which a heat generating material is embedded projects to the outside the metallic outer sleeve and, at the same time, a rear end side of the ceramic heater to which an electrode lead-out fitting is connected is positioned inside the metallic outer sleeve.
In the glow plug which is configured such that a positive electrode which is exposed on a rear end portion of the ceramic heater and the electrode lead-out fitting are connected with each other in the inside of the metallic outer sleeve, a heat-resistant insulating powder made of magnesium oxide (MgO) or the like is sealed in the inside of the metallic outer sleeve, and the heat-resistant insulating powder is brought into a highly dense state by swaging or the like, thus firmly fixing the electrode lead-out fitting.
In such a constitution which connects the positive electrode of the ceramic heater and the electrode lead-out fitting in the inside of the metallic outer sleeve, it is necessary to further reduce a diameter of the rear end portion of the ceramic heater and hence, breaking of the rear end portion must be sufficiently taken into consideration.    Patent document 1: Japanese Patent Laid-open Hei10-332149 (page 4 to 5, FIG. 1).